Zircon U-Pb analyses of 18 orthogneisses from the Clearwater and Priest River complexes of northern Idaho have identified important exposures of Neoarchean and Paleoproterozoic basement rocks in northwest Laurentia. All samples have ages that fall into two tightly defined age ranges: in the Neoarchean from 2.67 to 2.65 Ga and in the Paleoproterozoic from 1.88 to 1.84 Ga. The Neoarchean orthogneisses show no evidence of older components, whereas some of the Paleoproterozoic orthogneisses have xenocrystic zircon cores with ages overlapping with those of the Neoarchean gneisses. Collectively, these two packages of rocks represent the two main periods of crust formation in northwest Laurentia, one wholly juvenile in the Neoarchean, and the other a mixture of juvenile Paleoproterozoic and inherited Neoarchean components.
Based on existing data, the ages of orthogneisses in the Clearwater and Priest River complexes are identical and probably represent one continuous crustal block; we refer to this basement domain as the Clearwater block. The Paleoproterozoic 1.86 Ga magmatism described here is distributed throughout the northwest Laurentian margin and does not coincide with the proposed trend of the Great Falls tectonic zone. Therefore, a model of a single linear arc preserved within the Great Falls tectonic zone is inadequate in describing the majority of the known 1.86 Ga crust in the region.